3 week old chick, wing/feather issue

[MAJMooreFOB]

We've had this little girl for three weeks now, she is slower to develop than her sister. Today we noticed her left wing has a (for lack of better word) sore. She has been in pine shavings with 9 others all the same age yet she is the only one that looks like this. Any ideas?

 

[Wyorp Rock]

Looks like she may have a broken blood feather on the wing? Hard to tell.
Is the area on her side just dried blood or is there a wound as well?

Likely being plucked by another chick. Observe behavior.